Fix FieldTextInput lack of Change event

FieldTextInput was not triggering a Change event and this prevented an
onWorkspaceChanged event from firing in the BlocklyPanel. This commit
separates the semantics of the text and value getter/setters so that
updates to the text in onHtmlInputChange handler do not prevent an
event being triggered in setValue() after the user commits the change.

core/field_textinput.js

@@ -66,6 +66,13 @@ Blockly.FieldTextInput.prototype.CURSOR = 'text';
  */
 Blockly.FieldTextInput.prototype.spellcheck_ = true;
 
+/**
+ * The value of the field.
+ * @type {?string}
+ * @private
+ */
+Blockly.FieldTextInput.prototype.value_ = null;
+
 /**
  * Close the input widget if this input is being deleted.
  */
@@ -92,6 +99,18 @@ Blockly.FieldTextInput.prototype.setValue = function(newValue) {
     }
   }
   Blockly.Field.prototype.setValue.call(this, newValue);
+  if (newValue !== null) {
+    this.value_ = newValue;
+  }
+};
+
+/**
+ * Get the value of this field.
+ * @returns {?string}
+ * @override
+ */
+Blockly.FieldTextInput.prototype.getValue = function() {
+  return this.value_;
 };
 
 /**